Solid oxide fuel cell or solid oxide electrolyzing cell and method for operating such a cell

ABSTRACT

A solid oxide fuel cell or a solid oxide electrolyzing cell includes a) a plurality of cathode-anode-electrolyte units, each CAE-unit having a first electrode for an oxidizing agent, a second electrode for a combustible gas, and a solid electrolyte between the first electrode and the second electrode and b) a metal interconnect between the CAE-units. The interconnect having a first gas distribution element and a gas distribution structure for the combustible gas, wherein the first gas distribution element is in contact with the second electrode of the CAE-unit, and a second gas distribution element having channels for the oxidizing agent and including separate channels for a tempering fluid. The channels for the oxidizing agent are in contact with the first electrode of an adjacent CAE-unit, and the first gas distribution element and the second gas distribution element being electrically connected.

CROSS-REFERENCE TO RELATED APPLICATION

This application is the U.S. national phase of PCT Application No.PCT/EP2013/062051 filed on Jun. 11, 2013, which claims priority to EPPatent Application No. 12171565.0 filed on Jun. 11, 2012, thedisclosures of which are incorporated in their entirety by referenceherein.

TECHNICAL FIELD

The invention concerns a solid oxide fuel cell or a solid oxideelectrolyzing cell. The invention further concerns a method foroperating a solid oxide fuel cell or a solid oxide electrolyzing cell.

BACKGROUND OF THE INVENTION

A fuel cell is a device that generates electricity by a chemicalreaction. Among various fuel cells, solid oxide fuel cells (SOFC) use ahard, ceramic compound of metal (e.g., calcium or zirconium) oxide as anelectrolyte. Typically, in solid oxide fuel cells, an oxidizing agent,such as O₂, is reduced to oxygen ions (O²—) at the cathode, and acombustible gas, such as H2 gas, is oxidized with the oxygen ions toform water at the anode.

A SOFC fuel cell comprises a stack of fuel cell units. A SOFC fuel cellunit consists of two major components, a cathode-anode-electrolyte-unit,also referred to as CAE-unit, and an interconnect, having the form of acassette in some cases. The interconnect serves to connect the CAE-unitof one fuel cell unit electrically to the CAE-unit of another fuel cellunit, so that the electrical power that each CAE-unit generates can becombined. Such interconnects have in planar High-Temperature Fuel Cells(SOFCs) the function to electrically connect the CAE-unit as well as totransport the combustible gas and the oxidizing agent to the respectiveelectrodes of the CAE-unit.

Because the interconnect is exposed to both the oxidizing and reducingside of the CAE-unit at very high temperatures of about 500° C. up to1100° C., interconnects are one of the critical issues of solid oxidefuel cells. For this reason, ceramics have in the past been moresuccessful in the long term than metals as interconnect materials.However, these ceramic interconnect materials are very expensive ascompared to metals. While metal interconnects are relatively easy tofabricate and process, they generally suffer from high power degradationrates partly due to formation of metal oxides, such as Cr₂O₃, at aninterconnect-anode/cathode interface during operation. Nickel- andsteel-based alloys are becoming more promising as lower temperature(600-800° C.) SOFCs are developed.

U.S. Pat. No. 7,632,586 B2 discloses an interconnect for a combustiblegas and an oxidizing agent. The planar CAE units are positioned oneabove the other with interconnecting layers formed as planar metalplates arranged in between neighboring CAE units. The respectivepassages for fuel and oxidant are formed in the anode and cathodelayers.

Due to the very high operating temperatures of a SOFC fuel cell stackthe effects of thermal expansion and the thermomechanical behavior ofthe CAE unit and the interconnect structures for supplying the CAE unitwith the reactants and conducting the reactants away therefrom have tobe taken into account. In particular, the gas distribution structuresmay undergo some creep, which affects the distribution of flows in thefuel cell. Moreover, the electrodes and interfaces tend to degrade assoon as excessive temperatures are reached.

U.S. Pat. No. 6,670,068 B1 discloses a SOFC fuel cell stack. Thus aplurality of CAE units are in electrically conductive contact with aninterconnector, the interconnector comprising a contact plate and afluid guiding element which is formed as shaped sheet metal part andconnected to the contact plate in a fluid-tight manner by welding orsoldering. Thereby the contact plate defines a fluid chamber having acombustible gas or an oxidizing agent flowing through it duringoperation of the fuel cell unit. The shaped sheet metal part is disposedwith a plurality of corrugations giving it a wave-like structure. Thewave-like structure as such may compensate for some of the thermalexpansion of the CAE unit and of the fluid guiding element in operation.However due to the local contact of the wave peaks or wave troughs withthe respective electrode, the fluid guiding element has to follow thethermal expansion of the electrode. If the fluid guiding element doesnot have sufficient elasticity the strain due to thermal expansion isintroduced into the electrode. The electrodes are formed from solid,brittle ceramics. Thus, if a high strain is introduced into theelectrodes, cracks may be formed, which will ultimately destroy theelectrode. In addition the welding or soldering connection providedbetween the fluid guiding element and the anode also contributes to thestiffness of the construction. In particular if materials having adifferent coefficient of thermal expansions are used, the strains mayfinally lead to damages of the electrode and may damage the cellmembrane concerned. In particular the flow of reactants may be alteredor direct mixing of them can occur if the cell membrane is broken,leading to spontaneous combustion. Thus locally hot spots may form,which may induce local thermal expansion and thus further development oflocal stress.

Therefore, there is a need for development of improved interconnects forsolid oxide fuel cells, addressing one or more of the aforementionedproblems, so that more reliable and efficient solid oxide fuel cells areachieved.

Thus it is an object of the invention to improve existing SOFC fuelcells, to make them more reliable, and to allow cheaper manufacturing.

SUMMARY OF THE INVENTION

The object of the invention is obtained by a solid oxide fuel cellcomprising the features of claim 1.

The object of the invention is in particular obtained by a solid oxidefuel cell, comprising

-   a) a plurality of cathode-anode-electrolyte units, each CAE-unit    comprising    -   a first electrode for an oxidizing agent,    -   a second electrode for a combustible gas,    -   and a solid electrolyte between the first electrode and the        second electrode, and-   b) a metal interconnect between the CAE-units, the interconnect    including:    -   a first gas distribution element comprising a gas distribution        structure for the combustible gas, wherein the first gas        distribution element is in contact with the second electrode of        the CAE-unit, and    -   a second gas distribution element comprising channels for the        oxidizing agent and comprising separate channels for a tempering        fluid, wherein the channels for the oxidizing agent are in        contact with the first electrode of an adjacent CAE-unit, and        the first gas distribution element and the second gas        distribution element being electrically connected.

The object of the invention is further in particular obtained by amethod for operating a solid oxide fuel cell or a solid oxideelectrolyzing cell, the cell comprising

-   a) a plurality of cathode-anode-electrolyte units and-   b) a metal interconnect between the CAE-units, the interconnect    including:    -   a first gas distribution element comprising a gas distribution        structure for the combustible gas, and    -   a second gas distribution element comprising channels for the        oxidizing agent and comprising separate channels for a tempering        fluid,        wherein at least a first and a second control temperature are        measured,    -   the first temperature being the temperature of the tempering        fluid entering the second gas distribution element or any        representative temperature measured on the tempering fluid inlet        side of the fuel cell,    -   and the second temperature being the temperature of one of the        exit temperature of the tempering fluid leaving the second gas        distribution element, the temperature of the fuel cell stack or        any representative temperature measured on the tempering fluid        outlet side of the fuel cell,        wherein the amount of tempering fluid supplied to the second gas        distribution element is controlled based on a temperature        difference of the first and second temperature.

The invention in this application is described by disclosing a solidoxide fuel cell (SOFC). The embodiment according to the invention couldalso be used as a solid oxide electrolyzing cell (SOEC). By describing asolid oxide fuel cell herein, also the same embodiment used as anelectrolyzing device is meant also, and is also covered by the claims,unless specifically specified.

A solid oxide fuel cell according to the invention comprises a pluralityof cathode-anode- electrolyte units and a metal interconnect betweeneach of the CAE-units, the interconnect including:

-   -   a first gas distribution element comprising a gas distribution        structure for the combustible gas, wherein the first gas        distribution element is in contact with the CAE-unit, and    -   a second gas distribution element comprising channels for the        oxidizing agent and comprising separate channels for a tempering        fluid, wherein the channels for the oxidizing agent are in        contact with the adjacent CAE-unit, and wherein the first gas        distribution element and the second gas distribution element        being electrically connected. The interconnect therefore serves        to electrically connect a CAE-unit with an adjacent CAE-unit. In        addition, the interconnect further serves to transport the        combustible gas and the oxidizing agent. The interconnect        further comprises a channels for a tempering fluid, to cool        (mostly SOFC units) or the heat (mostly SOEC units) in        particular the CAE-unit.

In a preferred embodiment the first gas distribution element for a fuelcell (or an electrolyzing device) comprises a first layer and a secondlayer, said first and second layers are disposed with a gas distributionstructure forming a pattern for combustible gas. An inlet opening isprovided for the combustible gas to the gas distribution structureformed between the first layer and the second layer by said pattern andan outlet opening is provided for discharging reaction products from thegas distribution structure. A supporting layer, which is the second gasdistribution element, is provided for tempering the first layer forregulating the temperature of the reactant fluid or of the first layerand thus provide a homogeneous temperature distribution to the reactantfluid flowing through the gas distribution structure.

If the expression “or” is used in this application for combining twoalternatives, both the combination of both alternatives as well as thepresence of only one of the alternatives is to be understood. If it isnot specifically referred to a fuel cell, the features may be applied toeither fuel cells or electrolyzing devices.

Advantageously, the second gas distribution element, also referred to asthe supporting layer, is arranged on a side of the first gasdistribution element, the second gas distribution element extending on aside opposite to the first gas distribution structure. In particular,the first and/or second gas distribution structure can be configured asa channel system.

According to an embodiment, the second gas distribution element isdisposed with a plurality of corrugations. In particular, thecorrugations can form a plurality of passages in particular extendingparallel to each other. According to a variant the passages can opentowards the first layer, first gas distribution element. In particular,the passages can have one of a wave profile, a zig-zag profile or aprofile of trapezoidal cross-section. Alternatively or in combinationtherewith, the passages are shaped as closed channels, in particulartubular channels. Such tubular channels can in particular have acircular or a rectangular cross-section. The passages can extendsubstantially along the main direction of flow from the inlet opening tothe outlet opening. According to an embodiment the first gasdistribution element comprises a first and a second layer, whereby thesecond layer is a homogenizing element, which has apertures, which havea length and a width, with the length being greater than the width andthe length extending in a transverse direction to the main direction offluid flow. The pattern for the reactant fluid flow of said first layercan comprise at least one of a plurality of channels, three dimensionalstructures, such as pins, grid structures, foam structures. In a furtherembodiment the channels of the first layer are at least partially beobstructed by at least a bar element. According to an embodiment atleast some of the second apertures of the second layer are shaped asperforations, wherein the length of the second apertures is greater thanthe width of the bar element, so that the second apertures may bypassthe bar element.

A method for operating a solid oxide fuel cell thus comprises the stepsof letting a combustible gas flow through the first gas distributionelement, and an oxidizing agent along the second gas distributionelement. The first gas distribution element comprises a first layer anda second layer, said first and second layers are disposed with a patternfor a fluid flow, such that a first gas distribution structure is formedon at least one of the first or second layers. An oxidizing agent flowsin the second gas distribution element, the second gas distributionelement being electrically connected with the first gas distributionelement. The second gas distribution element is advantageously providedfor tempering the first layer, the combustible gas and the CAE unit.

If the method is applied to a solid oxide fuel cell, the combustible gasfluid provides a source for electrons to the cathode-anode-electrolyteunit and the oxidizing agent provides charge carrying ions to thecathode-electrolyte anode unit, such that the charge-carrying ions cancross the electrolyte and that an electrochemical reaction can beperformed at the electrolyte whereby electrons are freed to generate anelectric current. The electrons are supplied or discharged via theinterconnect providing an electrically conductive pathway between twoCAE-units.

The first and second gas distribution elements for a solid oxide fuelcell (SOFC) enable the appropriate distribution of the combustible gasand the oxidizing agent, thus gas on the fuel electrode of the fuel cellas well as proper electrical contact with the latter. This inventionthus concerns also the gas distribution element and its construction ina fuel cell or electrolyzing device stack. The fuel cell is usuallyconfigured as a fuel cell stack composed of a plurality of unit cells.The unit cells are combined in a modular fashion into such a fuel cellstack as to achieve the voltage and power output level required for theapplication. The stacking thus involves connecting multiple unit cellsin series via electrically conductive interconnects. A unit cell can bein particular configured as a cell membrane.

For a solid oxide fuel cell or an electrolyzing device it is essentialthat the combustible gas is homogeneously distributed over the fuelelectrode in order to maximize its efficiency and guarantee a reliableoperation. This requires that the gas distribution structure of thefirst gas distribution element, such as a channel system or a porousstructure, presents a homogeneous resistance to gas flow, thus an evenpressure drop. For the channel structures, this requires usually theprovision of a very precise geometry, involving very tight fabricationtolerances and incurring therefore high manufacturing costs.

The ceramic gas diffusion layer which is placed on either side of thesolid oxide fuel cell which, in turn, is sandwiched between two metallicinterconnects reduces the cost of the overall stack by making it lesscomplex and less expensive to manufacture as far as materials areconcerned.

According to an embodiment, the gas distribution structure of the firstlayer is at least partially obstructed by at least a bar element. Thebar element is to be considered as an obstacle to the fluid flow throughthe gas distribution structure of the first layer. The bar element canbe any type of barrier or throttle element, which forces the fluid flowto deviate from proceeding in the main direction of fluid flow, or thatcreates a local restriction of the hydraulic diameter of the flowchannels.

At least some of the first or the second apertures of the second layercan be shaped as perforations, in particular as holes. The first andsecond layers thus form a gas distribution element, which is composed ofat least one sheet metal.

In the gas distribution element, the at least one sheet metal layerforms a channel structure facing the perforated layer. The particularityof the perforated layer is to present a series of elongated holesextending substantially perpendicular to the fuel distribution channelsand allowing mixing the gas of several channels in the near environmentat regular intervals along the flow direction.

Advantageously the length of the perforations is greater than the widthof the bar element. Either the first or second reactant fluid can thuspass over the obstacle formed by the bar element and therefore the flowdeviates from the main direction of flow allowing for a mixing of thestream through one channel with streams passing through adjacentchannels. According to an embodiment, a portion of the apertures, inparticular shaped as perforations, has a length greater than the widthand either the length or the width extends in the main direction offluid flow. In particular the width of the first apertures extends inthe main direction of fluid flow or the length of the second aperturesextends in the main direction of fluid flow. The gas distributionstructure arranged on the first layer and at least one the firstapertures and second apertures are in fluid contact.

A supporting layer, forming an additional layer, can be provided for aneven distribution of either one of the first or second reactant fluidsonto an electrode. According to an embodiment a plurality of inletopenings for the respective reactant fluid are provided on at least oneof the first and second layers. By providing a plurality of inletopenings, a more even distribution of fluid flow can be obtained. Afurther advantage is the more even distribution of heat, thus allowingmaking efficient use of the entire reactive surface provided by the CAEunit.

Furthermore gas distribution structures forming the pattern for fluidflow, in particular at least some of the first or second apertures canbe manufactured by stamping, or etching. According to an alternativeembodiment, the supporting layer forms a monolithic piece with the firstlayer. According to an embodiment, the first layer comprises a firstsheet containing perforations and a second sheet forming the base layer.The supporting layer can be arranged on the opposite side of the baselayer or of the first layer.

Furthermore, the invention concerns fuel cell or an electrolyzing devicecomprising a gas distribution element according any one of the precedingembodiments.

In particular, the total open area of the first apertures is at least20% of the total contact surface of the negative electrode of thecathode-anode-electrolyte unit, preferably at least about 30% of thetotal contact surface, most preferred at least about 50% of the totalcontact surface. Thereby a lateral distribution of the gas flowingthrough the gas distribution element is obtained, which allows for amore homogeneous fluid distribution and consequently of a more uniformfluid temperature.

Primary applications for SOFCs are in the fields of remote power,distributed power generation, Combined Heat and Power (CHP), AuxiliaryPower Units (APUs) for trucks, buses, and ships, portable power andefficient biogas conversion.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and advantages of the invention will be morefully understood and appreciated from the following description ofcertain exemplary embodiments of the invention taken together with theaccompanying drawings, in which like numerals represent like compounds.The invention is described in detail in combination with a fuel cell. Itis obvious that the invention also covers an electrolyzing device.

FIG. 1 is a schematic view of a SOFC system,

FIG. 2 is an isometric view on a first gas distribution element,

FIG. 3 a cross-sectional view of a unit cell according to a firstembodiment of the invention,

FIG. 4 an explosion view of the unit cell of FIG. 3,

FIG. 4A an enlarged view of a second gas distribution element,

FIG. 4B an explosion view of a further embodiment of a first gasdistribution element,

FIG. 4C an explosion view of a further embodiment of a first gasdistribution element,

FIG. 4D a further embodiment of a second layer, the homogenizing layer,

FIG. 4E a further embodiment of a second layer, the homogenizing layer,

FIG. 4F a further embodiment of a unit cell comprising a first and asecond gas distribution element,

FIG. 4G a section through the second gas distribution element,

FIG. 4H a further embodiment of a second gas distribution element,

FIG. 5 a partial top view of two neighboring layers of a gasdistribution element,

FIG. 6A a partial top view of a perforated layer of a gas distributionelement,

FIG. 6B a section along line A-A of FIG. 6A,

FIG. 6C a section along line B-B of FIG. 6A,

FIG. 6D an enlarged section of an ideal gas distribution element alongline C-C of FIG. 4 but without the supporting layer,

FIG. 6E a section of a gas distribution element without a homogenizinglayer,

FIG. 6F an enlarged section along line C-C of FIG. 4 of a gasdistribution element comprising a homogenizing layer,

FIG. 6G a schematic view showing ideal conditions of flow of acombustible gas through a gas distribution element,

FIG. 6H a schematic view showing real conditions of flow of acombustible gas through a gas distribution element,

FIG. 6I a schematic view showing real conditions of flow of acombustible gas through a further gas distribution element,

FIG. 6K a section of a gas distribution element without a homogenizinglayer,

FIG. 6L a section of a similar gas distribution element as shown in FIG.6K but the gas distribution element comprising a homogenizing layer,

FIG. 7A a schematic view showing ideal conditions of flow of acombustible gas through a gas distribution layer of a fuel cell unit,

FIG. 7B a schematic view showing optimal designed real conditions offlow of the combustible gas through a fuel cell unit,

FIG. 7C a schematic view showing conditions of flow of the combustiblegas through a fuel cell unit according to the prior art,

FIG. 7D a view on a stack of fuel cell units with a flow according toconditions shown in FIG. 7B,

FIG. 7E a view on a stack of fuel cell units with a flow according toconditions shown in FIG. 7C,

FIG. 8 a section though a plurality of consecutive layers of fuel cellunits of a stack,

FIG. 8A a detailed section view of FIG. 8,

FIG. 8B a section of a schematic side view of a fuel cell stack,

FIG. 8C a section of a schematic side view of a further embodiment of afuel cell stack.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a solid oxide fuel cell (SOFC) system 100 according to theinvention. The solid oxide fuel cell system comprises a casing 101,which contains a fuel cell stack 103 being composed of a plurality offuel cell units 50, whereby the fuel cell units are herein also termedunit cells 50. The casing rests on a basement 102. The fuel cell systemor balance of plant includes a heat exchanger 106 for heating thereactants as well as reactant preparation units for providing thereactants in the correct composition and the correct flow rate to thefuel cell, which are not shown in the drawings. The stacks are disposedwith reactant discharge elements 104, 105.

The stack can be configured as shown in U.S. Pat. No. 7,632,586 B2,where a particular electrode contacting and gas distribution structureis applied. In the prior art, a stack based on this technology has beendeveloped for remote and micro-Combined Heat and Power (CHP)applications of about 1 kW. It is characterized by low pressure dropsand can achieve power densities of 1 kW/1 or 400 mW/cm² with electricalefficiencies of above 45%. The stacks can be fuelled with reformednatural gas, reformate gas or hydrogen. This stack manifolds the airexternally and the fuel internally and recovers the fuel exhaust stream.The exhaust stream can be used in post combustion or recycled forreforming (given adapted balance of plant). The use of U.S. Pat. No.7,632,586 B2 improves the thermal cycling tolerance of the stack,avoiding additional performance degradation due to thermal cycling.

With two recent prototypes combining the present invention with thetechnology disclosed U.S. Pat. No. 7,632,586 B2, an improved performancewas measured. A maximum fuel conversion of 94% was attained withefficiencies reaching 61% using hydrogen as fuel and 69% using methane.Moreover, up to 50 thermal cycles were attained without significantdamage on a short stack of that combined type. This is far above earlierresults based on the sole handling of reactant flow as disclosed in U.S.Pat. No. 7,632,586 B2.

For the distribution of combustible gas a first gas distribution element10 is foreseen which is depicted in detail in FIG. 2. An interconnect 40comprises a first gas distribution element 10 and a second gasdistribution element 4. The interconnect 40 is usually arranged betweentwo neighboring cathode-anode electrolyte units 5. Under a unit cell 50,a unit comprising a cathode-anode-electrolyte unit 5, and theinterconnect 40 is to be understood.

The first gas distribution element 10 is used for providing at least thecombustible gas to the respective electrode.

The second gas distribution element 4 is used for providing the reactantcontaining oxygen, which means the oxidizing agent to the respectiveelectrode.

The first gas distribution element 10 disclosed in FIG. 2 comprises afuel inlet 16 and a fuel outlet 18, so that the fuel provided by inlet16 flows within the first gas distribution element 10 in lineardirection of flow 9 from the inlet 16 to the outlet 18. In FIG. 2 afirst layer 2is arranged below a second layer 3.

For the operation as a fuel cell, the reactant containing oxygen issupplied to the positive oxygen electrode 51 acting as a cathode.

For an operation of the unit cell 50 as an electrolyzing device, thereactant containing oxygen is supplied to the same positive oxygenelectrode 51 acting as an anode

In an advantageous embodiment the gas distribution element 10 is usedfor providing a combustible gas to the negative electrode 53 of the CAEcathode-anode-electrolyte unit 5. The interconnect 40 further comprisesa second gas distribution element 4 comprising fluid conducting channelsfor the reactant containing oxygen, allowing to put in contact thereactant containing oxygen with the positive oxygen electrode 51 of aneighboring CAE cathode-anode-electrolyte unit 5.

In most cases the oxygen-containing reactant is air, however also pureoxygen or an oxygen containing gas may be supplied to the interconnect40. The second reactant, the combustible gas, usually contains anymixture of H₂, CO, H₂O, CO₂, methane, ammonia, other hydrocarbons oroptional diluents.

In a preferred embodiment, the combustible gas is distributed inside thegas distribution element 10. The negative electrode 53 of the CAEcathode-anode-electrolyte unit 5 is thus facing a second layer 3of thegas distribution element 10.

The first gas distribution element 10 can also be used for anelectrolyzing device operating in the inverse way. For the operation asa fuel cell, the reactant containing oxygen is supplied to the positiveoxygen electrode acting as a cathode.

For an operation of the unit cell as an electrolyzing device, thereactant containing oxygen is supplied to the positive oxygen electrodeacting as an anode.

The interconnect 40 combines two essential functions of the fuel cellstack 103: it accomplishes current collection from the electrodes 51,53and it manifolds the reactant, in particular the fuel and also theoxygen containing gas between and on the CAE cathode-anode-electrolyteunit 5.

As disclosed in FIG. 3 the interconnect 40 thus allows to integrate thegas distribution of the unit cell 50, allowing the use of thin, notmachined metallic sheets as shown by reference numbers 1,2,3 and/or 4,which for example may be manufactured by stamping, punching,roll-forming, embossing or etching, which means cheap manufacturing,instead of expensive, structured bi-polar plates. The base layer 1and/orthe first layer 2and/or the second layer 3and/or the supporting layer4can be manufactured by stamping, embossing, punching or etching or byhot pressing, or other processes such as powder metallurgy. The firstgas distribution element 10 may be manufactured such that the base layer1, the first layer 2, the second layer 3or any combination thereof arejoined together by any suitable bonding technique such as welding,brazing or reactive bonding, or any combination thereof, for electricalcontacting and/or sealing. In a similar way the second gas distributionelement 4 may be manufactures by forming the supporting layer or bycombining the supporting layer with the base layer 1.

The proposed fuel cell stack 103 includes according to a preferredapplication between 1 and 100 unit cells 50, corresponding to 16-5000 Wnominal electrical power.

The embodiment shown in FIG. 3 shows a sectional view of an arrangementof a unit cell 50 comprising a cathode-anode-electrolyte unit 5 and ainterconnect 40, the interconnect comprising the first gas distributionelement 10 and the second gas distribution element 4.

The first gas distribution element 10 according to the embodiment shownin FIG. 3 consists of a base layer 1, a second layer 3and a first layer2. The cathode-anode-electrolyte unit 5 comprises a first electrode 51,a second electrode 53 and an electrolyte 52 sandwiched between the firstand second electrodes 51, 53. The unit cell 50 further comprises lateralseals 31, which provide a gas tight seal for the edges of thecathode-anode electrolyte unit 5 and the contacting layers 55 and thegas distribution element 10. The unit cell 50 further comprises thesecond gas distribution element 4 for supplying the first reactant fluidcontaining oxygen to the first electrode 51. The second reactant fluidcomprising the fuel is supplied to the second electrode 53 above thefirst layer 2respectively the second layer 3.

FIG. 4 shows an explosion view of a fuel cell unit 50 comprising aCAE-unit 5 and an interconnect 40. The CAE unit 5 comprises a firstelectrode 51, a second electrode 53 and an electrolyte 52 sandwichedbetween the first and second electrodes 51, 53. Usually a ceramic and/ormetallic gas diffusion layer 54,55 is arranged on both sides of theelectrodes 51,53, which is not shown in FIG. 4, but which for example isshown in FIG. 8A.

The example of a first gas distribution element 10 shown in FIG. 4comprises a base layer 1, a first layer 2and a second layer 3; saidfirst 2 and second layers 3 are disposed with a gas distributionstructure 11 forming pattern for a fluid flow. The first layer 2,disclosed in FIG. 4, defines a flow pattern by a number of channels 13laying one beside the other, so that the combustible gas entering thefirst layer 2may flow in the main direction of flow 9. The channels 13extend in linear direction. The channels 13 preferably start on one sideof the first layer 2at an entrance side 2 b, also called inlet, and thechannels 13 preferably end on the other side of the first layer 2, atthe exit side 2 c, also called outlet, whereby the entrance side 2 b isconnected with a combustible gas supply 9 a, and whereby the outlet 2 cis fluidly connected to an exhaust gas exit 9 b. FIG. 4 also shows thesecond gas distribution element 4, which in the example shown is acorrugated sheet of metal, having channels 20, as disclosed in FIG. 4A.In FIG. 3 a sectional view of the fuel cell unit 50 along line C-C canbe seen. The first gas distribution element 10 is explained. The firstlayer 2comprising a plurality of spaced channel bars 2 a formingchannels 13 there between. As disclosed in FIG. 4 the first layer 2maycomprise further channels 12, 14 extending in linear direction, andwhich fluidly connect the channels 13 with the inlet 2 b respectivelythe outlet 2 c.

The second layer 3is a homogenizing element comprising apertures 15which fluidly connect at least two channels 13 laying one beside theother, to compensate and to homogenize the amount of fluid in therespective channels 13. In FIG. 3 an aperture 15 is disclosed fluidlyconnecting three channels 13. The second layer 3has first apertures 15which are configured as rectangular openings having a length 28 and awidth 29. The length is greater than the width. The length 28 extendstransversely to the main direction of fluid flow 9; the width 29 extendsin the main direction of fluid flow 9. The second layer 3may also havesecond apertures 6 which have a length 7 and a width 8, with the length7 being greater than the width 8 and the width 8 extending in atransverse direction to the main direction of fluid flow 9.

The first layer 2, also called channel layer, has a plurality of inletchannels 12, a plurality of consecutive channels 13 and a plurality ofoutlet channels 14. Consecutive channels 12 and 13 are separated by abar element 23. Consecutive channels 13 and 14 are also separated by abar element 23. The bar elements 23 are necessary to connect the bars 2a.

These second apertures 6 of the second layer 3form channel-likestructures, which are arranged in particular rectangular or inclined tothe inlet channels 12 arranged in the first layer 2. This has theadvantage, that the fluid flowing inside the channels 12, 13, 14 of thefirst layer 2may be directed by a bar element 23, which is part of thefirst layer 2, arranged on the first layer towards the aperture 6 of thesecond layer 3, as disclosed in FIG. 2. The aperture 6 thus forms afluid passage between consecutive channels 12 and 13, or betweenconsecutive channels 13 and 13, or between consecutive channels 13 and14 by traversing the bar element 23 trough aperture 6. Whenever thefluid flows over the bar element 23 it enters the aperture 6 above thebar element 23 and is distributed into a consecutive channel 13,respectively 14. One advantage of such an embodiment is that the firstlayer 2and the second layer 3can be manufactured very cheap by usingthin metal sheets.

Advantageously each inlet channel 12 is continued with a consecutivechannel 13 and an outlet channel 14. These channels 12, 13, 14 may havethe same cross-section and may be arranged one behind each other.Advantageously a plurality of inlet channels 12, consecutive channels 13and outlet channels 14 are foreseen as disclosed in FIG. 4. Each of theinlet channels 12 may be arranged parallel to the correspondingneighboring inlet channel 12, the same may apply also to the consecutivechannels 13 or outlet channels 14.

The first layer 2and the second layer 3may be formed on separate sheetsas shown in FIG. 4; however, they may also be combined into a singlesheet. Furthermore the first layer 2may be manufactured as a sheethaving perforations corresponding to the channels 12, 13, 14 and beingarranged beside a base sheet 1 forming the base for the channels 12, 13,14. This solution can be advantageous for the manufacture of thechannels. Furthermore a considerable variety of shapes is available forthe perforations. The perforations may be conveniently punched out ofthe sheet, laser cut or also etched or formed as lost inserts that areremoved after casting or molding the layer. Thus foreseeing a base layer1and the second layer 3as separate sheets may provide a simplificationin manufacture or the application of a greater variety of manufacturingmethods to manufacture the layers 1, 2, 3.

Furthermore two inlet openings 16, 17 are provided for the reactantcomprising the fuel, which is the combustible gas, to enter the gasdistribution element 10. In addition two outlet openings 18, 19 may beprovided for the fluid reaction product, which is the waste gas, toleave the gas distribution element 10.

In a preferred embodiment the second gas distribution element 4 isarranged on the side of the base layer 1and is connected with the baselayer 1. FIG. 4 shows the flow path of the oxidizing agent O, thesupporting layer having channels 20 on both sides, which are channels 20a, 20 b. FIG. 4A shows an enlarged view of a preferred structure of thesupporting layer 4, whereby the flow path of the oxidizing agent O issplit be the channels 20 a, 20 b in two flow paths O1, O2, so that eachpath flowing in a channel 20 along one side of the supporting layer 4.

FIG. 4B shows a further embodiment of a gas distribution element 10. Thebase layer 1and the first layer 2defining the flow pattern being made ofone single part. In this embodiment there is no need for bar elements 23holding the bars 2 a, because the bars 2 a are connected with the baselayer 1, so that the plurality of channels 13 extend in lineardirection, one beside the other, whereby the channels 13 start at theentrance side 2 b and end at the exit side 2 c, so that the channelsfluidly connect the entrance side 2 b with the exit side 2 c. Becausethe bar element 23 are not needed, also the apertures 6 to fluidlyconnect consecutive channels 12,13,14 are not needed in the second layer3, as disclosed in FIG. 4B.

FIG. 4C shows a further embodiment of a gas distribution element 10. Thefirst layer 2comprises a porous structure 2 d, such as a piece ofmetallic foam or metal mesh, whereby the porous structure being arrangedon the base layer 1. The first layer 2defining a flow path starting atthe entrance side 2 b and ending at the exit side 2 c, so that theporous structure fluidly connects the entrance side 2 b with the exitside 2 c, so that the porous structure defining a flow path extending inlinear direction.

FIG. 4D shows a further embodiment of a second layer 3, a homogenizerelement. In contrast to the embodiment disclosed in FIG. 4B, showing asecond layer 3of rectangular shape, FIG. 4D shows a second layer 3ofcircular shape. In contrast to the embodiment disclosed in FIG. 4B,showing a first layer 2of rectangular shape with parallel extendingchannels 13, a first layer adapted to the second layer 3disclosed inFIG. 4D would have a circular shape and comprising channels 13 extendinglinear in radial direction, starting in the center at the fuel inlet 2b, which is at the same location as the fuel inlet opening 16, andending at the periphery, where a fuel outlet 2 c is arranged thatpreferably totally surrounds the first and second layer 2,3, so that thecombustible gas 9 a within the first gas distribution element 10 flowsin radial direction. Only a few of the channels 13 are shown in FIG. 4D.The second layer 3 comprises a plurality of apertures 15 extending incircumferential direction, the apertures 15 transversely crossing thechannels 13 of the first layer 2, so that some of adjacent channels 13are fluidly connected by respective apertures 15. A first gasdistribution element 10 comprising a first and second layer 2,3 asdisclosed in FIG. 4D is therefore of circular shape. To build a circularfuel cell unit 50, a circular cathode-anode-electrolyte unit 5 can bearranged on top of the second layer 3, and a supporting layer 4could bearranged below the first layer 2, so that a fuel cell unit 50 isachieved, similar to the one disclosed in FIG. 4, but with radiallyextending channels 13 in the first layer 2, and radially extendingchannels 20 in the supporting layer 4. The first layer 2arranged beneaththe second laser 3 may also be a three dimensional structure such aspins, grid, mesh structures or foam structures, the first layer 2havinga circular shape and a direction of fluid flow 9 a, 9 b, 9 c extendingin radial, in particular in linear direction from an inlet 2 b to anoutlet 2 c, and the first apertures 15 of the second layer 3extending incircumferential direction. In an advantageous embodiment there are nochannels within the foam structure, but the porous structure of the foamallows a fluid to flow within the foam so that the fluid is flowing in adirection of fluid flow 9 a,9 b,9 c within the first layer 2.

FIG. 4E shows a further embodiment of a second layer 3of rectangularshape comprising apertures 15 extending in circular direction. Incontrast to the second layer 3disclosed in FIG. 4D, the apertures 15 ofthe second layer 3disclosed in FIG. 4E are arranged in three groups 9 xof apertures 15 of similar dimensions, whereby these groups 9 x aredisplace respective to each other in circumferential direction. Such anarrangement of apertures 15 increases the homogenizing effect on theflux of the fuel passing the channels 13. The second layer 3disclosed inFIG. 4E comprises a circumferential fuel outlet 2 c collecting the wastegas to the fuel outlet ports 18/19 so that the fuel in the first layer2may first flow in radial direction 9 u and then in direction 9 v to thefuel outlet 2 c.

FIG. 4F shows a further embodiment of a fuel cell unit 50 comprising aCAE-unit 5 and a interconnect 40. The interconnect 40 comprising a firstgas distribution element 10 and a second gas distribution element 4. Thefirst gas distribution element 10 consisting of a base plate 1 on whichthe channels 13 are fixed, and consisting of a sealing layer 3 d with anaperture 3 e. The aperture 3 e is adapted to the size of the CAE-unit 5,so that the CAE-unit may be introduced into the aperture 3 e, so thatthe CAE-unit 5 can be arranged just above the channels 13. The seconddistribution element 4 is built as already disclosed in FIG. 4. Incontrast to the embodiments disclosed in FIGS. 4, 4B and 4C, the firstgas distribution element 10 disclosed in FIG. 4F does not comprise asecond layer 3, which means does not comprise a homogenizing layer 3.

FIG. 4G shows a section along the line D-D of FIG. 4F in detail, wherebyFIG. 4G also includes a CAE-unit 5 arranged below the second gasdistribution element 4, which is not shown in FIG. 4F. FIG. 4G shows thecorrugated sheet of metal in detail, which is arranged between theCAE-unit 5 and the base layer 1. The second gas distribution element 4is connected by connections 4 c such with the base layer 1, thatelectricity may be flow between the second gas distribution element 4and the base layer 1. They for example may be welded together at theconnections 4 c. The corrugated sheet has a wave profile, a zig-zagprofile or a profile of trapezoidal cross-section. The corrugationshaving a pitch 20 g, the pitch 20 g being in the range of 2 mm to 8 mm.A small pitch 20 g has the advantage, that electricity flowing betweenthe corrugated sheet and a position at the electrolyte 52 where theelectrochemical reaction takes place undergoes a lower ohmic resistance,because there is a higher density of contacting points between thecorrugated sheet and the CAE-unit 5. On the other hand a small pitchcauses the channels 20, 20 a, 20 b to be very small, which increases theflow resistance of the fluid flowing in the channels 20.

The sheet metal thickness of element 4 is in the range of 0.3-1 mm, morepreferably between 0.3 . . . 0.6 mm, and most preferably 0.5 mm.

In a preferred embodiment, the channels 20 a for the oxidizing agenthave a cross sectional area 20 f, and the channels 20 b for thetempering fluid have a cross sectional area 20 e. The ratio of the twocross sectional areas 20 e, 20 f is in the range of 1:2 to 2:1,preferably 1:1.

In a preferred embodiment the channels 20 a for the oxidizing agent andthe channels 20 b for the tempering fluid have a height in the rangebetween 1 to 5 mm.

In a preferred embodiment the corrugations have a gradient angle (α) ofat least 45°, more preferably larger than 60°.

In a preferred embodiment, the channels 13 of the first gas distributionelement 10 extend from a fuel inlet side 2 a to a fuel outlet side 2 bthereby defining a direction of flow 9 of the combustible gas within thefirst gas distribution element 10, whereby the channels 20 a, 20 b ofthe second gas distribution element 4 either extend substantially alongthe main direction of flow 9 or extend substantially perpendicular tothe main direction of flow 9. As disclosed in FIG. 4G, in a preferredembodiment, the channels 20 b for the tempering fluid are in contactwith the first gas distribution element 10, which means the channels 20b are facing the first gas distribution element 10, respectively thebase layer 1, so that there is a direct contact of the tempering fluidflowing in channels 20 b with base layer 1.

In a preferred embodiment the corrugations form a plurality of channels20 a, 20 b extending parallel to each other.

In a preferred embodiment the second gas distribution element 4 isconnected to the first gas distribution element 10 in such a way thatthe channels 20 b for the tempering fluid are shaped as closed channels,comprising only a entrance end 20 c and an exit end 20 d. This isachieved by connecting the corrugated sheet in such a way with the baselayer 1, that each channel 20 b forms a gas tight channel between itsentrance end 20 c and its exit end 20 d.

In an advantageous embodiment the second gas distribution element 4consists of at least two parts, the at least two parts being separatedfrom each other by a split 4 b having a gap width of at least 0.3 mm.FIG. 4H discloses such a second gas distribution element 4 consisting offour parts, and having two splits 4 b.

FIG. 5 shows a partial top view of the first and second layers 2, 3 of afirst gas distribution element 10 of a third embodiment in a view aspartial cut from the top side of the gas distribution element 10. Thecross sectional view of a portion of the first layer 2shows some of thechannels 13, one beside the other and separated by a channel bar 2 a andsome of the consecutive outlet channels 14, separated by the bar element23 from the channels 13. The first layer 2is arranged behind the secondlayer 3. The second layer 3contains first apertures 15 having length 28and a width 29 with the length 28 extending transverse, in thisembodiment perpendicular, to the main direction of fluid flow 9.

FIG. 6A shows a partial top view of a perforated second layer 3of a gasdistribution layer 10 according to any of the first, second or thirdembodiments of the invention, comprising first apertures 15 andunderlying channel bars 2 a. FIG. 6B, a section along line A-A of FIG.6A, shows the cathode-anode-electrolyte unit 5, the first layer2comprising channel bars 2 a, the second layer 3 and the base layer 1.The base layer 1and the first layer 2are manufactured from distinctsheets. FIG. 6C shows a section along line B-B of FIG. 6A. As adifference to FIG. 6B the section traverses a row of apertures 15,therefore the second layer 3is interrupted by the apertures 15.Furthermore the parallel extending channels 13 in the first layer 2areshown.

FIG. 6D shows a section along line C-C of FIG. 4, without the supportinglayer 4, in detail. The first gas distribution element 10 consisting ofthree layers, the base layer 1, on top of which the first layer 2isarranged, defining the flow pattern comprising a plurality of channels13 separated by bars 2 a extending parallel in flow direction 9. Thesecond layer 3, which is the homogenizing layer, is arranged on top ofthe first layer 2. The second layer 3comprising first apertures 15extending perpendicular to the flow direction 9. In the embodimentshown, the first apertures 15 extend over three channels 13, to fluidlyconnect the three channels 13, so that a fluid exchange 9 z might takeplace between the three combustible gas streams 9 a, 9 b, 9 c; 9 d,9 e,9f and through the first apertures 15. FIG. 6D shows an ideal first gasdistribution element 10 in that each of the channels 13, K1 . . . K6have identical width and identical height and identical flow resistance,so that each of the combustible gas streams 9 a,9 b,9 c,9 d,9 e,9 f haveabout the same flow rate and about the same gas composition andresulting diffusive flux of reactants and reaction products to thecathode-anode-electrolyte unit 5, so that minor or no fluid exchange 9 zbetween the gas streams 9 a,9 b,9 c;9 d,9 e,9 f takes place within thefirst apertures 15. In addition to the fluid exchange 9 z between thethree combustible gas streams 9 a, 9 b, 9 c; 9 d,9 e,9 f as described,the first apertures 15 have also the effect, that within the firstaperture 15, which is facing the cathode-anode-electrolyte unit 5, thegas composition leaving the streams 9 a,9 b,9 c; 9 d,9 e,9 f are mixedand homogenized, before entering the cathode-anode-electrolyte unit 5.Therefore the gas composition is homogenized before entering thecathode-anode-electrolyte unit 5, which guarantees that unit 5 isprovided with a sufficient amount of reactive gas, even if one or eventwo of the gas streams 9 a,9 b,9 c; 9 d,9 e,9 f provide not sufficientgas. The cathode-anode-electrolyte unit 5 and the second gas contactingand gas diffusion layer 55 arranged on top of the second layer 3are onlyschematically shown.

FIG. 6F shows a section along line C-C of FIG. 4 in detail. In contrastto FIG. 6D showing an ideal gas distribution element 10, FIG. 6F shows acommon arrangement in which the channels K1 . . . K6 have slightlydifferent shapes, for example a different width, and therefore differentflow resistance, which causes the effect, that the gas streams 9 a,9 b,9c,9 d,9 e,9 f have different flow rates. The advantage of the secondlayer 3, the homogenizing layer, is, due to the first apertures 15fluidly connecting some of the channels K1,K2,K3; K4,K5,K6, a fluidexchange 9 z occurs between the gas streams 9 a,9 b,9 c,9 d,9 e,9 f sothat the difference in flow rate between the gas streams 9 a,9 b,9 c,9d,9 e,9 f is reduced, which means the gas streams are homogenized, sothat the gas composition and resulting diffusive flux of reactants andreaction products of the combustible gas F along thecathode-anode-electrolyte unit 5 is harmonized.

FIG. 6E shows the embodiment according to FIG. 6F, but without thesecond layer 3. In absence of the homogenizing layer, the gascomposition and resulting diffusive flux of reactants and reactionproducts of the combustible gas F along the cathode-anode-electrolyteunit 5 may strongly vary, depending on the different shapes of thechannels K1 . . . K6. One advantage of the second layer 3, thehomogenizing layer, therefore is, that the first layer 2can bemanufactures in a cheaper way, because the effect of variances inchannel width and/or channel height on the gas streams 9 a, 9 b, 9 c, 9d, 9 e, 9 f can be compensated by the homogenizing layer, thus allowingto manufacture a cheap and reliable gas distribution element 10.

FIG. 6G shows a top view of the first gas distribution element 10disclosed in FIG. 6D, showing six channels K1 . . . K6 extending inparallel direction, three channels K1,K2,K3; K4,K5,K6 being fluidlyconnected by apertures 15, whereby each of the gas streams 9 a,9 b,9 c,9d,9 e,9 f have the same flow rate. A plurality of apertures 15 arearranged and spaced apart in flow direction 9.

FIG. 6H shows a top view of the first gas distribution element 10disclosed in FIG. 6F, showing six channels K1 . . . K6 extending inparallel direction, three channels K1,K2,K3; K4,K5,K6 being fluidlyconnected by apertures 15, whereby gas streams 9 a,9 b,9 c,9 d,9 e,9 fentering the gas distribution element 9 have different flow rates. Aplurality of apertures 15 are arranged and spaced apart in flowdirection 9, whereby in each of the apertures 15 a fluid exchange 9 zmay occur between the gas streams 9 a,9 b,9 c; 9 d,9 e,9 f so that thedifference in flow rate between the gas streams 9 a,9 b,9 c; 9 d,9 e,9 fis reduced. The first gas distribution element 10 comprises theapertures 15 therefore ensure that none of the channels K1 . . . K6 isdeprived with gas, and that the cathode-anode-electrolyte unit 5 willnot suffer from local depletion of fuel. The homogenizing layer3therefore has the effect, that damaging of the fuel cell unit 50 due tolack of combustible gas in some areas of the fuel cell unit 50 isavoided. Moreover, in the apertures 15 a homogenization of compositionsby diffusion and convection takes place. This reduces further the riskof having one area of the cell damaged by local depletion of combustiblegas, even in the event of having one of the channels K1 . . . K6 e.g.clogged by any unwanted residue. In that case, the gases can circumventthe clogged part of channel through the apertures 15 and the gas diffusethrough the aperture 15 above the clogged channel to the electrode.

FIG. 6I shows a top view of a further embodiment of a gas distributionelement 10, showing six channels K1 . . . K6 extending in paralleldirection, the channels K1,K2,K3; K4,K5,K6 being fluidly connected byapertures 15, whereby gas streams 9 a,9 b,9 c,9 d,9 e,9 f entering thegas distribution element 9 have different flow rates. In contrast to theembodiment disclosed in FIG. 6H, the apertures 15 in the embodimentaccording to FIG. 6I have different length 28, and therefore may fluidlyconnect two, three, four or even more parallel extending channels K1 . .. K6. In addition, consecutive apertures 15 spaced apart in flowdirection 9 may be shifted perpendicular to the direction of flow 9and/or may have different length 28, therefore connecting differentchannels K1 . . . K6.

FIG. 6L shows a section along line C-C of FIG. 4C in detail, the firstlayer 2comprising a porous structure 2 d through which the combustiblegas 9 flows. In contrast to the first gas distribution element 10disclosed in FIG. 6F comprising channels K1 . . . K6, the gas flow ismore diffuse in the porous layer disclosed in FIG. 6L, therefore the gasstreams 9 a,9 b,9 c,9 d,9 e,9 f disclosed in FIG. 6L show only the fuelflow intensity (magnitude) flowing in flow direction 9. The effect ofthe second layer 3, the homogenizing layer, is similar to the effectdisclosed in FIG. 6F, in that the second layer 3causes a fluid exchange9 z between the gas streams 9 a,9 b,9 c,9 d,9 e,9 f, if the gas streamshave different gas composition. Therefore the second layer 3homogenizesthe flow rate of the gas various streams 9 a,9 b,9 c,9 d,9 e,9 f in theporous structure of first layer 2. Therefore the gas composition andresulting diffusive flux of reactants of the combustible gas F along thecathode-anode-electrolyte unit 5 is harmonized.

FIG. 6K shows the embodiment according to FIG. 6L, but without thesecond layer 3. In absence of the homogenizing layer 3, the gascomposition and resulting diffusive flux of reactants of the combustiblegas F along the cathode-anode-electrolyte unit 5 may strongly vary,depending on flow resistance in the porous first layer 2, similar to theeffect disclosed in FIG. 6E.

FIG. 7A is a schematic view showing ideal conditions of flow of acombustible gas through a gas distribution layer of a fuel cell unit 50,whereby the fuel cell unit 50 in this example comprises twelve channels13, laying one beside the other, and whereby the arrows indicate theflux of the combustible gas in the respective channels 13. The x-axis ofthe coordinate system shows the flux in the respective channel 13 in themain direction of flow 9. The y-axis shows the channel number of twelvechannels K1-K12, arranged one beside the other, as indicated in FIG. 3.FIG. 7D shows a stack of ten fuel cell units 50, each fuel cell unit 50having twelve channels 13, the channel number disclosed in FIG. 7A, 7Bcorresponds to a channel as shown in the fuel cell stack of FIG. 7D.FIG. 7B is a schematic view showing optimal real conditions of flow ofthe combustible gas through a fuel cell unit 50, whereby, due toconstruction compromises in the gas manifolding, the flux of combustiblegas is lower on the lateral channels 1 and 12 close to the casing, thusthe flow velocity close to the casing of the fuel cell unit 50 havingthe lowest value.

FIG. 7D is a view on a stack of fuel cell units 50, with each fuel cellunit 50 having an identical flow according to conditions shown in FIG.7B. Therefore, the average flux F1 to F10 of each of the ten fuel cellunits 50 is the same.

FIG. 7C is a schematic view showing real conditions of flow of thecombustible gas through a fuel cell unit according to the prior art,thus a very inhomogeneous distribution of flow velocity. Theinhomogeneous distribution of flow velocity occurs for example fromproduction tolerances when manufacturing the fuel cell unit 50. FIG. 7Cshows the same designed flow field as in FIG. 7B, but with importantdeviations from the designed due to for example manufacturingtolerances. This is a typical problem in prior art. The deviations aredifferent from one distribution element to another, depending on itsmanufacturing. In the example disclosed in FIG. 7C the channel havingthe lowest gas flux is the number 5, but it can be any other channel inanother distribution element. This minimum flux may lead to local fuelstarvation and consequently to performance limitations, to localoverheating of the fuel cell stack, or even to cracks in theelectrolyte, anode or cathode materials, leading possibly to a breakageof the CAE unit 5 and possibly to fuel and oxidant mixing and parasiticcombustion, thus a premature severe damage of the stack or at least ofparts thereof.

FIG. 7E is a view on a fuel cell stack comprising ten fuel cell units 50as disclosed in FIG. 7C. The individual fuel cell units 50 presentrandom deviations, with the location of the minimum channel flow varyingfrom one to another, therefore the average flow velocity in each of thefuel cell units 50, indicated by the length of arrows F1 . . . F10, israndomly distributed. These random deviations have a twofold effect:first, the total flux per fuel cell unit varies among units 50 due todifferent resistances to the fluid flow, and second, the hence cumulateddeviation from an average flux per channel (7A, ideal case) becomesconsequently more important. For this reason, in prior art,compensations have to be introduced, by correcting the entering flow atthe unit cell manifold, by sorting out batches of unit cells with narrowpressure drops, by increasing the specifications for tolerances, orfurther by reducing the fuel conversion rate to reduce the operationalrisk. All this has an effect on costs on the production of the stack andon the efficiency of the system. Moreover, FIG. 7E shows that in fuelcell stacks according to the prior art, the flow conditions inneighboring fuel cell units 50, respectively the flow conditions inneighboring gas distribution elements 10 may vary significantly.

Modeling and experimental work on solid oxide fuel cells has shown howimportant the homogeneity of the fuel distribution and the arrangementof flows are for the performance and reliability of fuel cells. FIG. 7Arepresents such an ideal case for air and fuel flowing in the same or inthe opposite direction. Due to fabrication processes, some compromisesare often required, which result in gas distributions that slightlydiffer from the ideal case as shown in FIG. 7B. The most recent researchincludes the study of the effect of fabrication tolerances or non-idealcomponent properties on performance and reliability, thus allowingassessing the suitability of industrial processes or specific designsfor the desired performance and reliability.

The work made by Cornu and Wuillemin (Impact of random geometricdistortions on the performance and reliability of an SOFC (2011) FuelCells, 11 (4), pp. 553-564) shows in particular how the quality of fueldistribution depends on the tolerances of the depth of the channels inthe gas distribution structures. The depth of the channels rangesusually from 0.2 mm to the 1-2 mm scale, and their width vary more oftenfrom 1 to 2 mm. Depths in the range of 0.5 mm are often found. In suchcases, depth variations of 0.05 mm around the targeted value alreadyhave a very important impact on flow distribution. An example of suchdeviation is given in FIG. 7C. Even if depth variations of 0.05 can beachieved by appropriate fabrication techniques, the space between thecathode-anode-electrolyte unit 5 and first gas distribution element 10can also vary depending on the contact layers used in between. Thecumulated depth variations for the effective channel sections aretherefore very difficult to maintain in the above-mentioned range ofdeviations. Last, but not least, the contacting layers or channels maycreep with time, which will in any case lead to a poor fuel distributionwith time.

As unit cells 50 are stacked on top of each other, the defects of theindividual elements will cumulate, leading to an even increaseddeviation of flows in operation which is shown by the case of FIG. 7E.

As exactly the same amount of fuel is converted in all unit cells 50 ofthe fuel cell stack, thus a common current flow is obtained, so that theareas of the unit cells 50 presenting a low fuel flow are exposed to therisk of fuel starvation when the fuel conversion is increased. As alarge conversion is required to reach high performance, a poor fueldistribution will lead to performance limitations or to the damaging ofone unit cell due to fuel starvation.

As there is hardly any sign for the operator that part of the fuel cellstack is suffering from starvation unless it is already too late, thiskind of problem is of large importance from an industrial and operativepoint of view.

FIG. 8 is a section though a plurality of consecutive fuel cell units 50forming a fuel cell stack 103, each fuel cell unit 50 comprising aCAE-unit 5 and a interconnect 40, the interconnect 40 comprising a firstgas distribution element 10 and a second gas distribution element 4 inaccordance with the embodiment as shown in FIG. 4.

Thus, the cross-section of the fuel channels 13 is given and determinedby the geometry of the channel structure of the first layer 2and thesecond layer 3being a perforated plate. The second layer 3being ahomogenizing element. Any optional additional contacting layer usedbetween the latter and the cathode-anode-electrolyte unit 5 will have noinfluence on the flow. Moreover, the geometry of holes 15 on theperforated plate, the second layer 3, allows a fluid exchange and mixingof the fluid along the fluid path of several channels 13, the channels13 laying one beside the other along the fuel path, hence creatingnear-isobars among channels at those locations, and hence creatingsuitable average flux among channels 13. Thanks to this, any deviationof geometry in any channel 13 along the fluid flow path of thecombustible gas within the first gas distribution element 10 iscorrected by allowing the combustible gas to flow between adjacentchannels 13, hence using the averaging effect to homogenize therespective reactant respectively combustible gas fluid flow.

FIG. 8A is a detailed section view of FIG. 8 showing two gasdistribution elements 10 with corresponding supporting layers 4 indetail. One cathode-anode-electrolyte unit 5 can be seen in the middleof FIG. 8A, whereby a supporting layer 4is contacting the first gascontacting and gas diffusion layer 54 on top of thecathode-anode-electrolyte unit 5, and whereby the second layer 3, thehomogenizing layer, is contacting the second gas contacting and gasdiffusion layer 55 on the bottom of the cathode-anode-electrolyte unit5. The second layer 3providing first apertures 15 extending over threechannels 13, to fluidly connect the three channels 13, so that a fluidexchange 9 z homogenizes the combustion gas F entering thecathode-anode-electrolyte unit 5.

The supporting layer 4has a corrugated shape, that allows to split theflow path of the oxidizing agent O into two separate flow paths O1, O2,with channels 20 b, 20 a, the flow paths O1 of channels 20 b being theoxidizing agent providing the cathode-anode-electrolyte unit 5 with theoxidizing agent O3. The flow path O2 of channels 20 a serves as atempering agent to cool or heat the base layer 1and/or thecathode-anode-electrolyte unit 5.

FIG. 8B shows in a section view a schematic side view of a fuel cellstack 103 comprising three fuel cell units 50, each comprising aninterconnect 40 and a CAE-unit 5, and each interconnect 40 comprising afirst gas distribution element 10 and a second gas distribution element4. The oxidizing agent O is provided on one side to all of the secondgas distribution elements 4, the oxidizing agent O is then split to formtwo separate flow paths O1, O2 along the second gas distributionelements 4, and the two separate flow paths O1, O2 are combined afterleaving the second gas distribution element 4, and the flow paths of allsecond gas distribution elements 4 are also combined to one single flowpath that exits the fuel cell stack 103. FIG. 8B also discloses a bloweror compressor 21 to feed the oxidizing agent O, and temperature sensors22 a, 22 b to measure the temperature of the oxidizing agent O enteringrespectively leaving the interconnect 40. The blower or compressor 21and the temperature sensors 22 a, 22 b are connected by cables 23 with acontrol unit 23, which is only schematically shown. Additionaltemperature sensors or other sensors or actuators could be arranged andcould be connected with the control unit 23 to control the operation ofthe fuel cell stack.

FIG. 8C shows in a section view a schematic side view of a furtherembodiment of a fuel cell stack 103 comprising three fuel cell units 50,each comprising an interconnect 40 and a CAE-unit 5, and eachinterconnect 40 comprising a first gas distribution element 10 and asecond gas distribution element 4. The flow paths O1, O2 are completelyseparated, and the oxidizing agent O is provided on one side to only thechannels 20 a which define the flow path O1 of the oxidizing agent. Atempering fluid O4 is provided on one side to only the channels 20 bwhich define the flow path O2 of the tempering fluid. The flow paths O1,O2 also leave the second gas distribution element 4 as separate paths.FIG. 8C also discloses a blower or compressor 21 to feed the temperingfluid O4, and temperature sensors 22 a, 22 b to measure the temperatureof the tempering fluid path O2 entering respectively leaving theinterconnect 40. The blower or compressor 21 and the temperature sensors22 a, 22 b are connected by cables 23 with a control unit 23, which isonly schematically shown. Additional temperature sensors or othersensors or actuators could be arranged and could be connected with thecontrol unit 23 to control the operation of the fuel cell stack.

A fuel cell stack 103 as disclosed in FIG. 8B or 8C may be operated byvarious methods. Some advantageous methods are now described in furtherdetails.

The fuel cell stack 103 may be operated by a method for operating asolid oxide fuel cell or a solid oxide electrolyzing cell, the solidoxide fuel cell comprising

-   a) a plurality of cathode-anode-electrolyte units 5 and-   b) a metal interconnect 40 between the CAE-units 5, the interconnect    40 including:    -   a first gas distribution element 10 comprising a gas        distribution structure 11 for the combustible gas, and    -   a second gas distribution element 4 comprising channels 20 a for        the oxidizing agent and comprising separate channels 20 b for a        tempering fluid,        wherein at least a first and a second control temperatures T1,        T2 are measured,    -   the first temperature T1 being the temperature of the tempering        fluid entering the second gas distribution element 4 or any        representative temperature measured on the tempering fluid inlet        side of the fuel cell,    -   and the second temperature T2 being the temperature of one of        the exit temperature of the tempering fluid leaving the second        gas distribution element 4, the temperature of the fuel cell        stack or any representative temperature measured on the        tempering fluid outlet side of the fuel cell,        wherein the amount of tempering fluid supplied to the second gas        distribution element 4 is controlled based on a temperature        difference of the first and second temperature T1,T2.

In a preferred method step for operating a solid oxide fuel cell or asolid oxide electrolyzing cell the amount of tempering fluid is suppliedto the second gas distribution element 4 and is controlled based on amaximal, a minimal or a nominal temperature difference of the first andsecond temperature T1, T2.

In a further preferred method step for operating a solid oxide fuel cellor a solid oxide electrolyzing cell the amount and the temperature T1 ofthe tempering fluid which is supplied to the second gas distributionelement 4 is controlled such that the first and second controltemperatures T1, T2 are maintained within defined minimum and maximumvalues.

In a further preferred method step for operating a solid oxide fuel cellthe flow rate of the oxidizing agent is maintained in excess of thestoichiometric flow required for the electrochemical reaction, in such away, that the oxygen partial pressure of the oxidizing agent at theoutlet of the channels 20 a is more than 5%, and preferably more than10% of the total pressure of the oxidizing agent.

In a further preferred method step for operating a solid oxide fuel cellor a solid oxide electrolyzing the oxidizing agent and the temperingfluid circulate in strictly separated flow paths O1, O2.

In a further preferred method step for operating a solid oxideelectrolyzing the tempering fluid heats the second gas distributionelement 4.

In a further preferred method step for operating a solid oxideelectrolyzing cell a carrier gas is added into the flow path O1 of theoxidizing agent to collect the generated oxygen, whereas the flow rateof the carrier gas is controlled such as to maintain the oxygen contentin the carrier gas leaving the interconnect 40 within a given range.

In a further preferred method step for operating a solid oxideelectrolyzing cell the carrier gas is circulated and oxygen is extractedfrom the carrier gas leaving the interconnect 40, to separately collectoxygen enriched gas.

In a further preferred method step for operating a solid oxideelectrolyzing cell pure oxygen is separately collected as it leaves theinterconnect (40).

FIG. 4 shows a cathode-anode-electrolyte unit 5 having a length 3 a anda width 3 b, which defines a contacting surface 3 c through which thecathode-anode-electrolyte unit 5 contacts the second layer 3. The secondlayer 3comprises the same contacting surface 3 c. The first apertures 15of the second layer 3are arranged within the contacting surface 3 c. Ina preferred embodiment the total area of all first apertures 15 is atleast 20% of the total area of the apertures 15, 6 and others foundwithin the surface 3 c. To provide an even more equal distribution ofthe combustible gas along the contacting surface 3 c, in a morepreferred embodiment the total area of all first apertures 15 is atleast 20% of the contacting surface 3 c, and most preferably about 30%and most preferably between 40% to 50%.

In a preferred embodiment the CAE-unit 5 has a length 3 a along thedirection of flow 9 and has a width (3 b), wherein the ratio of thelength 3 a to the width 3 b preferably being greater than 1, morepreferably being greater than 1.5 and most preferably being greater than2.

The first apertures 15 disclosed are shown with rectangular shape. Thefirst apertures 15 can also have other shapes, such as an ellipticshape. The second layer 3could also comprise a plurality of firstapertures 15 of different shapes, such as for example rectangular andelliptic shapes on the same second layer 3.

An advantageous method for homogenizing a combustible gas in a first gasdistribution element 10 of a fuel cell is, that the first gasdistribution element 10 comprises a first layer 2connecting a fuel inlet2 b with a fuel outlet 2 c, whereby the fuel is flowing in a directionof flow 9, within the first layer 2, in particular in linear direction,and the first gas distribution element 10 comprises a second layer3comprising first apertures 15, the first apertures 15 extending intransverse direction with respect to the direction of flow 9, whereinthe combustible gas flowing through the first layer 2enters the firstapertures 15 so that the combustible gas is homogenized within the firstapertures 15, and wherein the first apertures 15 are contacting acathode-anode-electrolyte unit 5, so that the combustible gas fromwithin the first apertures 15 is provided to thecathode-anode-electrolyte unit 5. In an advantageous method step, atleast some of the combustible gas homogenized within the first apertures15 flows back into the first layer 2.

In a further advantageous method step, the first layer 2comprises aplurality of channels 13 arranged one beside the other and connectingthe fuel inlet 2 b with the fuel outlet 2 c, the first apertures 15extending in transverse direction with respect to the channels 13 andfluidly connecting at least two channels 13 arranged one beside theother, wherein the combustible gas, flowing through the respectivechannels 13, enters the first aperture 15, so that the combustible gasof the respective channels 13 is homogenized within the first aperture15.

In an advantageous method step at least some of the combustible gashomogenized within the first apertures 15 flows back into the respectivechannels 13 of the first layer 2or is exchanged between the respectivechannels 13 of the first layer 2.

In an advantageous method step at least some the first apertures 15extend perpendicular to the direction of flow 9 so that the pressure ofthe combustible gas in the respective first aperture 15 is equalized, sothat the pressure of the combustible gas in the underlying first layer2or in the underlying respective channels 13 is equalized locally.

The structure was implemented in two stack designs according to U.S.Pat. No. 7,632,586 B2 and validated in operation. A maximum fuelconversion of 94% was attained with efficiencies reaching 61% usinghydrogen as fuel and 69% using methane. This is far above earlierresults based on the handling of reactant flow as disclosed in U.S. Pat.No. 7,632,586 B2.

Due to the exothermic reaction in the fuel cell unit, an active coolingof the fuel cell units 50 is therefore required, in particular during atransition phase, which can be principally achieved by air cooling, orby a combination of air cooling and internal cooling on the fuel side byusing the endothermic steam-reforming reaction of methane (SMR). This ishowever limited to the class of systems using steam-reformed methane asfuel.

To limit temperature gradients and excessive temperature differences inthe CAE-unit 5 and in the gas distribution structures, a properdistribution of the cooling air in the unit cell 50 is required. Tolimit temperature differences, a large excess of cooling air is requiredwith respect to the amount that would be necessary for theelectrochemical reaction itself. This excess air implies additionallosses in the balance of plant, in particular due to the powerconsumption of the air blowers. These losses can however be reduced ifthe pressure drop in the stack is low, that means, if the gasdistribution structure for the air in the stack presents a lowresistance to the air flow. The fuel cell is therefore operated with anominal pressure difference between its oxidant stream inlets andoutlets of preferably less than 50 mbar, resp. 20, resp. 10 mbar.

A problem which should be avoided with fuel cell stacks is localtemperature peaks developing on the surface of an electrode, whichusually forms a planar layer. If such local temperature peaks occur, thereaction kinetics may be altered and a local hot spot may be formed.Such a hot spot is undesired because it involves a high strain on thematerials, by causing a local thermal expansion, which may lead tothermal stress, warpage, buckling or deformations of the layer materialsaffected. Due to the fact that the ceramics materials of the electrodesor the electrolyte are brittle, they may be subject to cracks andeventually break if subjected to substantial local temperaturevariations. The occurrence of such hotspot can be drastically reduced byincreasing the cooling air flow, and by proper design of the airdistribution structure that contacts the CAE unit and hence can serve asheat dissipating structure.

Furthermore, temperature gradients within the fuel cell unit can resultin inappropriate thermal stress at other critical locations than in theCAE unit, such as in the seals used around the cell and in the fuelmanifolds which distribute the fuel in the stack. This may lead todelamination of the seals and detrimental leaks, both leading possiblyto a local or complete breakage of the CAE unit.

It is possible to operate the fuel cell with reduced air flows, but withthe consequence to reach larger temperature differences between airinlet and outlet. The drawback of this situation is that the cold sidewill suffer from less-efficient electrochemical reactions, as most ofthe electrochemical processes are thermally activated. It is known thatsome electrode types, in particular some cathode materials, will degrademore severely with time in such conditions. On the other hand, thehotter end of the fuel cell will experience other types of degradationthat are thermally activated, e.g. the growth of oxide scales on metalparts.

A further important point for the performance of the fuel cell is thehomogeneity of temperatures perpendicularly to the main direction 9 offuel flow. It seems that stacks having an air flow perpendicular to thefuel flow (so called cross-flow configuration) present importanttemperature differences perpendicularly to the fuel flow, leading to alack of fuel consumption along the cell on the colder side due toreduced electrochemical performances. This leads to the impossibility tooperate the stack at high fuel conversion rates, and hence to reducedefficiency. This problem can be partly circumvented by using thickinterconnectors to enhance the internal heat transfer, but at theexpense of weight and extra cost.

It is therefore preferable to operate the fuel cell with the fuel andair flows flowing in parallel or in opposite directions. Nevertheless,thermal gradients can occur on the lateral sides of the fuel stream, inthe vicinity of the boundaries of the stack, due to heat exchange withthe rest of the system. A similar problem of performance limitation maytherefore occur in such situations. Therefore, it is of interest tooperate the fuel cell with a large excess of air which will helpreducing such types of gradients. For the same purpose, it is ofinterest to build the fuel cell in such a way that the length of thereactive area of the cell along the fuel flow is greater than the width,that is, having an aspect ratio greater than one. In preferredconstructions, this aspect ratio is greater than 1.2, preferably greaterthan 1.5, and preferably greater than 2.

Therefore, it is of interest to reduce thermal gradients and temperaturedifferences within a fuel cell unit to increase the performance andlimit degradation.

Moreover, at low coolant flows larger temperature differences areexpected between the core of the stack and its boundaries e.g. first andlast fuel cell unit. This is not only detrimental for thermomechanicalreasons, but also due to the fact that the electrochemical performancewill vary from one location to another similarly disposed in the stack.As a maximum temperature has usually to be respected within the stack,e.g. to preserve sealing materials, some parts of the fuel cell willhave to be operated at lower temperatures than needed, with the resultthat the colder elements will operate at lower efficiencies and theoverall performance will be reduced.

Finally, the dynamic control of the fuel cell is enhanced when usinglarger coolant flows, since faster responses can be obtained and thecontrollability improved.

One drawback of the use of excess air however is the transport ofpoisoning species onto the air electrode. Especially volatile chromiumis known to be released by the metallic components situated upstream ofthe stack and transported into the stack by the air stream. The volatilechromium tends to deposit in the air electrodes by electrochemical andchemical reactions. In particular, volatile chromium reactsspontaneously with the strontium contained in the electrodes. Moreover,it can be deposited electrochemically as chromium oxide at theelectrode/electrode interface, hence reducing the number of reactingsites. Not only chromium, but also silicon, sulfur and other species areknown to further affect the durability of the air electrode.

Therefore, it would of particular advantage to have the possibility tooperate a fuel cell with increased air flows for homogeneous tempering,while having a low pressure drop on the oxidant stream to lower theauxiliary losses, and whereas only part of the air is put in contactwith the air electrode to prevent pollution.

Moreover, it would be advantageous to have the possibility to vary theratio between the coolant air and the reactive air, such as to operatethe fuel cell at optimal performance and reduced pollution of the airelectrode.

In the electrolysis mode, it can be of further advantage to separate theoxygen-rich gas obtained from the electrolysis reaction, from thetempering stream, in order to enable the storage of oxygen-rich gas asreaction product. In the electrolysis mode, the tempering stream is usedto heat up the stack, to provide heat to the endothermic electrolysisreaction when needed, and eventually to remove heat at some operatingpoints where the overall operation may become exothermic.

This separation is further advantageous for future applications wherethe fuel cell can be used reversibly in generator and electrolysis mode,e.g. for storage of renewable energy during peak production and laterre-use of reaction products in generator mode, including oxygen-enrichedgas as oxidant.

The invention claimed is:
 1. A solid oxide fuel cell, comprising; a) aplurality of cathode-anode-electrolyte units arranged in a stack, eachCAE-unit comprising; a first electrode for an oxidizing agent, a secondelectrode for a combustible gas, and a solid electrolyte between thefirst electrode and the second electrode, and b) a metal interconnectbetween adjacent CAE-units, across the entire stack, the metalinterconnect disposed between each pair of CAE-units, the interconnectincluding: a first gas distribution element comprising a gasdistribution structure for the combustible gas, wherein the first gasdistribution element is in contact with the second electrode of theCAE-unit, and a second gas distribution element comprising channels forthe oxidizing agent and separate channels for a tempering fluid, whereinthe channels for the oxidizing agent are in contact with the firstelectrode of an adjacent CAE-unit, and wherein the first gasdistribution element and the second gas distribution element beingelectrically connected, wherein the first gas distribution elementcomprises a planar base layer, a first layer and a second layer, whereinthe first and second layers are disposed with the gas distributionstructure, the second gas distribution element is arranged on the sideof the base layer of the first gas distribution element and forming asupporting layer, the separate channels for the tempering fluid are incontact with the baser layer of the first gas distribution element, thesecond gas distribution element comprises a plurality of corrugations,the corrugations forming a plurality of channels for the oxidizing agentand a plurality of separate channels for the tempering fluid, theplurality of channels extending parallel to each other, the corrugationsconsists of a corrugated sheet of metal, and a manifold is adapted suchthat the channels for the oxidizing agent in fluid communication with asource of oxygen containing gas and that the channels for the temperingfluid are in fluid communication with a separate tempering gas.
 2. Thefuel cell according to claim 1, wherein the channels for the temperingfluid are open towards the first gas distribution element.
 3. The fuelcell according to claim 1, wherein the channels for the oxidizing agentand the channels for the tempering fluid have one of a wave profile, azig-zag profile or a profile of trapezoidal cross-section.
 4. The fuelcell according to claim 1, wherein the second gas distribution elementis connected to the first gas distribution element, and wherein thechannels for the tempering fluid are shaped as closed channels,comprising only a entrance end and an exit end.
 5. The fuel cellaccording to claim 1, wherein the first gas distribution element extendsfrom a fuel inlet side to a fuel outlet side thereby defining a maindirection of flow of the combustible gas within the first gasdistribution element, whereby the channels of the second gasdistribution element either extend substantially along the maindirection of flow or extend substantially perpendicular to the maindirection of flow.
 6. The fuel cell according to claim 1, wherein thechannels for the oxidizing agent having a cross sectional area andwherein the channels for the tempering fluid having a cross sectionalarea, and wherein the ratio of the two cross sectional areas is in therange of 1:2 to 2:1.
 7. The fuel cell according to claim 1, wherein theCAE-units having a length along the direction of flow and having awidth, wherein the ratio of the length to the width being greaterthan
 1. 8. The fuel cell according to claim 1, wherein the corrugationshaving a pitch, the pitch being in the range of 2 mm to 8 mm.
 9. Thefuel cell according to claim 8, wherein the corrugations having agradient angle (α) of at least 45°.
 10. The fuel cell according to claim1, wherein the channels for the oxidizing agent and the channels for thetempering fluid having a height in the range between 1 to 5 mm.
 11. Thefuel cell according to claim 5, wherein the second layer, which is ahomogenizing element, has first apertures or second apertures wherein atleast some of the first apertures have a length and a width, with thelength being greater than the width and the length extending in atransverse direction to the main direction of fluid flow.
 12. The fuelcell according to claim 1, wherein the first gas distribution elementcomprises at least one of a plurality of channels.
 13. The fuel cellaccording to claim 12, wherein the channels of the first gasdistribution element are at least partially obstructed by at least a barelement, and wherein the second layer comprises apertures that bypassthe bar elements.
 14. The fuel cell according to claim 11, wherein thegas distribution structure of the first layer comprises a plurality ofgas distribution channels arranged one beside the other and connectingthe fuel inlet side with the fuel outlet side, wherein at least onefirst aperture, extending in transverse direction with respect to thegas distribution channels, have a length that at least two channelsarranged one beside the other are fluidly connected by the at least onefirst aperture.
 15. The fuel cell according to claim 14, wherein the gasdistribution channels extend in parallel to each other, and wherein thefirst apertures extend perpendicular to a plurality of channels.
 16. Thefuel cell according to claim 11, wherein the channels of the second gasdistribution element extend in a radial direction and the firstapertures extend in a circumferential direction.
 17. The fuel cellaccording to claim 1, wherein the first gas distribution element and/orthe second gas distribution element are manufactured by stamping,embossing, roll forming, punching or etching or by powder metallurgy.18. The fuel cell according to claim 1, wherein the interconnect or thefirst gas distribution element and/or the second gas distributionelement form a monolithic piece.
 19. The fuel cell according to claim13, wherein the second gas distribution element comprising at least twoparts, the at least two parts being separated from each other by a splithaving a gap width of at least 0.3 mm.